Sulfur Segregation and Surface Site Vacancy Compensation During Methanol to Methoxy Reactions on MoS 2
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.634
Sulfur Segregation and Surface Site Vacancy Compensation During Methanol to Methoxy Reactions on MoS2 Prescott E. Evans1, Hae-Kyung Jeong2, and Peter A. Dowben1 1
Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, NE 68588-0299, U.S.A
2
Department of Physics, Institute of Basic Science, Daegu University, Gyeongsan 712-714, Korea
ABSTRACT
Sulfur loss from the surface of MoS2(0001) is observed following the adsorption of methanol on MoS2 at 86 K and subsequent annealing of MoS2 near 300 K. This sulfur loss, at the MoS2 surface, leads to suppression of inverse photoemission features characteristic of the unoccupied states associated with MoS2. This sulfur loss is counteracted by further annealing to 350 K, as is evident in the temperature dependent sulfur to molybdenum integrated X-ray photoemission intensity ratios near 300 to 350 K. Upon further annealing to 350 K, inverse photoemission additionally indicates a reestablishment of characteristic features associated with the unoccupied states of MoS2. These results are indicative of sulfur segregation to the surface and compensation of surface vacancy sites.
INTRODUCTION Recent studies of the production of surface methoxy species, from MoS2(0001) exposure to methanol, found that the adsorbed methanol to surface methoxy conversion process generates sulfur vacancies on the MoS2(0001) basal plane [1]. This sulfur loss, from the surface of MoS2, as a result of methanol conversion to methoxy, was established utilizing scanning tunneling microscopy, photoemission, photoluminescence, and modeled by density functional theory (DFT) [1]. The adsorption of methanol on MoS2 at 110 K followed by annealing near 350 K or the adsorption of methanol on MoS2 at 350 K results in the formation of numerous point defects at the MoS2 basal plane [1]. Larger patch defects, nominally ~2 nm in size as well as line defects on the MoS2 sample surface became increasingly apparent with multiple cycles of methanol exposure and
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annealing [1]. The experimental results indicate a small but persistent activation energy for the reaction consistent with DFT predicted energetics. Literature provides strong evidence of the key role surface defects and vacancies play in catalysis on the MoS2 surface through modification of the surface electronic structure [2-4]. While selective desulfurization of MoS2(0001), to create surface vacancies, may alternatively be achieved through argon beam sputtering [5], utilization of a methanol to methoxy reaction may provide less destructive means in affecting bulk MoS2 features while allowing large scalability. The loss of surface vacancy sites, due to surface segregation of sulfur, would diminish catalytically active defects
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